<This seems very likely, the energy requirements of bloated blockchains are making it close to impossible for miners to stay profitable.>

This is absolutely not true, Bitcoin's network hashrate is growing continuously and has made near x10 since last year, we don't see how miners should continue to mine on an unprofitable chain. Mining difficulty adjust itself according to network hashrate to maintain constant block time extraction: the more is the hashrate, the higher difficulty will be and vice versa.

<Let's be honest, if mining Bitcoin ever becomes unprofitable for miners, then we can rest assured the blockchain will fail.>

This is the whole point on which blockchain relies on: the economic incentives are the basis of cryptoeconomy, even in PoS if token holders decide to not stake their tokens because it's not so profitable, then the corresponding blockchain will fail. Until now PoW has shown a lot of strenghts in terms of security and resiliency (if the network is big enough to make attacks too much expensive), as such as weaknesses in terms of resources wastage. PoS on the other hand is not expensive in terms of resources but there are trade-offs regarding decentralisation and security, which are main features of blockchain itself.

Best,
Black Swan